Injection molding is a commonly employed manufacturing technique for forming articles. An example of an article that is typically formed using injection molding is a plastic preform. A plastic preform can then be subsequently blow-molded into a plastic bottle.
An injection mold for making preforms (and other articles) typically includes one or more molding cavities for receiving molten plastic and forming the preforms. The cavities are usually defined between complementary cylindrically-shaped mold cavity inserts. The injection mold also includes a mold shoe, typically comprising a set of plates, within which the mold cavity inserts are arranged. The injection mold also includes a hot runner for communicating a flow of the molten plastic into the molding cavities.
The molten plastic injected into the cavities must be cooled to solidify the molten plastic so that the molded preform can be removed from the mold. It is desirable to cool the preform as quickly as possible so the preforms can be removed and a next injection cycle initiated with minimal time delay. As the mold cavity inserts are in direct contact with the molten plastic they become heated by it and need to be cooled. In the prior art, cooling of the mold cavity inserts was typically achieved by creating a cooling channel in an exterior surface of the mold cavity insert. Typically, such a channel is a spiral, but it can be any shape that directs a flow of coolant about the exterior of the mold cavity insert.
Those of skill in the art recognize that the mold cavity insert needs to withstand enormous pressures as the molten plastic is injected. Of note, however, since the cooling channel is integrally formed on the mold cavity insert, the stresses along the length of the mold cavity insert are non-uniform. Indeed, stress gradients at various points along the path of the channel of the mold cavity inserts can be observed. The stress is often the greatest where the cooling channel makes abrupt changes in direction.
In order to reduce the likelihood of catastrophic failure of the mold cavity insert due to excessive stress, the wall thickness of the mold cavity insert is increased to provide sufficient structural strength to withstand injection pressures. In at least certain prior art mold cavity inserts, the wall thickness of the mold cavity inserts are at least about 6.4 millimeters. However, since injection molds are typically made available with a standardized number and arrangement of molding cavities, a minimum permissible wall thickness of the mold cavity insert limits the maximum size of the preform or other article that can be formed therein.
FIG. 1 is a front view of a known injection mold cavity plate 60. The plate 60 includes a matrix of openings or bores 32 for receiving a plurality of mold stack assemblies (not depicted). In the present example, the matrix 60 is depicted as having four rows and eight columns. The size of the matrix 60 is determined by many factors such as the size of a molding machine (not depicted), the maximum size of the mold and the size of the articles to be made. The matrix 60 in the present example would accommodate a total of thirty-two mold stack assemblies.
FIG. 2 is a partial sectional view of a known injection mold 1 in use with the cavity plate 60 of FIG. 1. The mold 1 is depicted with a known molding stack assembly 53. The stack assembly 53 includes a mold cavity insert 54 with an outside diameter D, a gate insert 55, a neck ring pair 52, a core insert 57, and a locking ring 58 that are configured to cooperate to provide a molding cavity 59 along a set of molding surfaces disposed thereon. Molding cavity 59 thus provides a chamber within which a preform 4 can be formed. (Other configurations of cavity 59 would thus permit the formation of other articles other than preform 4). The mold shoe includes a cavity plate 60, a core plate 61, a stripper plate 62, and a slide pair 63.
In more detail, the core insert 57 is arranged on a front surface of the core plate 61 and retained thereon by the lock ring 58. The core plate 61 includes core coolant channels 76, 77 and 78 for connecting with a coolant channel configured within the core insert 57.
Core coolant channels 76, 77 and 78 further interconnect with core cooling tube 80. As can be seen, the entire inside of the molding cavity 59 extends along the molding surface of core insert 57. Coolant channels 70 and 17 provide coolant to one half of slide pair 63 and neck ring pair 52.
The mold cavity insert 54 is arranged within a bore in cavity plate 60. The gate insert 55 is arranged within a bore configured in a top portion of the mold cavity insert 54.
As can be seen, a substantial portion of the outside of the molding cavity 59 extends along the interior surfaces of mold cavity insert 54 and a smaller portion along gate insert 55.
Cavity plate 60 also includes coolant channels 75 for connecting with a coolant channel 90 that is configured around the periphery of mold cavity insert 54 to form a cooling circuit. The cooling channel 90 is defined on one side by the bore within cavity plate 60 and on the other side by a plurality of dividers 94 integrally formed in the wall of the mold cavity insert 54.
FIG. 3 is a perspective view of another known embodiment mold cavity insert 55 and gate insert 55 for use with the injection mold 1 of FIG. 2. The coolant channel 90 is substantially spiral, but is also characterized by two longitudinal channels 96. Each longitudinal channel 96 represents a substantial change in direction of the coolant flow in relation to the remainder of channel 90.
During injection molding to form the article, the presence of the cooling channel 90 in the mold cavity insert 54 and, particularly, the longitudinal channels 96, create greater stress concentrations in areas of the cavity insert 54 than would be the case if the coolant channel 90 was not present. Accordingly, the wall thickness T of the mold cavity insert 54 is clearly dictated to prevent mechanical failure of the mold cavity insert 54 in the areas of highest stress concentrations. As best seen in FIG. 2, the wall thickness T is typically the distance between the molding surface of the mold cavity 59 and the bottom of the cooling groove 90 in mold cavity insert 54. For a typical mold having a cavity insert of the type shown as mold cavity insert 54, the minimum wall thickness T for the mold cavity insert 54 is at least about 6.4 millimeters. As the diameter of the bore, of the openings 32, in a given cavity plate 60 is generally fixed, the mold is limited to forming parts that have an outside diameter that is limited by the wall thickness of the inert. If the thickness of the insert and its associated coolant channel can be reduced then parts having a larger diameter could be molded without having to modify the cavity plate.